The present disclosure relates to optical computing devices and, more particularly, to using multiple bandpass filters in optical computing devices to improve sensitivity and detectability.
Optical computing devices, also commonly referred to as “opticoanalytical devices,” can be used to analyze and monitor a sample substance in real time. Such optical computing devices will often employ a light source that emits electromagnetic radiation that reflects from or is transmitted through the sample and optically interacts with an optical processing element to determine quantitative and/or qualitative values of one or more physical or chemical properties of the substance being analyzed. The optical processing element may be, for example, an integrated computational element (ICE). One type of an ICE is an optical thin film interference device, also known as a multivariate optical element (MOE). Each ICE can be designed to operate over a continuum of wavelengths in the electromagnetic spectrum from the vacuum-UV to infrared (IR) ranges, or any sub-set of that region. Electromagnetic radiation that optically interacts with the sample substance is changed and processed by the ICE so as to be measured by a detector. The output of the detector can be correlated to a physical or chemical property of the substance being analyzed.
A traditional ICE (hereafter “ICE core”) includes first and second pluralities of optical thin film layers consisting of various materials whose complex indices of refraction and size (e.g., thickness) varies between each layer. An ICE core design refers to the substrate, number and thickness of the respective layers of the ICE core, and the complex refractive indices of the layers. The complex refractive index includes both the real and imaginary components of the refractive index. The layers are strategically deposited and sized so as to selectively pass predetermined fractions of electromagnetic radiation at different wavelengths configured to substantially mimic a regression vector corresponding to a particular physical or chemical property of interest of a substance of interest. Accordingly, an ICE core design will exhibit a transmission function that is weighted with respect to wavelength. As a result, the output light intensity from the ICE core conveyed to the detector may be related to the physical or chemical property of interest for the substance.
Optical computing devices that employ ICE cores generally incorporate a single bandpass element or filter to restrict the optical spectrum to regions of interest. This bandpass filter is often a commercially available wide optical bandpass filter. Such bandpass filters are normally chosen to isolate the optical region defined by a single short pass wavelength and a single long pass wavelength. However, in some applications these bandpass filters may encompass many wavelength regions where there is little information, and thereby decreasing sensitivity, detectability, and signal-to-noise ratios (SNR) of the optical computing device.